When voltage is applied on an organic electroluminescence device (hereinafter, occasionally referred to as an organic EL device), holes and electrons are respectively injected into an emitting layer from an anode and a cathode. The injected electrons and holes are recombined in an emitting layer to form excitons. Here, according to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%. In the classification according to the emission principle, in a fluorescent EL device which uses emission caused by singlet excitons, the limited value of an internal quantum efficiency of the organic EL device is believed to be 25%. On the other hand, in a phosphorescent EL device which uses emission caused by triplet excitons, it has been known that the internal quantum efficiency can be improved up to 100% when intersystem crossing efficiently occurs from the singlet excitons.
In a typical organic EL device, the most suitable device design has been made depending on fluorescent emission mechanism or phosphorescent emission mechanism. Particularly for designing a phosphorescent organic EL device, it has been known that simple application of a fluorescent device technique to a phosphorescent organic EL device does not provide a highly efficient phosphorescent organic EL device in consideration of a luminescence property of the phosphorescent organic EL device. The reasons are generally considered as follows.
First of all, since the phosphorescent emission is generated using triplet excitons, an energy gap of a compound for the emitting layer must be large. This is because a value of singlet energy (which means an energy gap between energy in the lowest singlet state and energy in the ground state) of a compound is typically larger than a value of triplet energy (which means an energy gap between energy in the lowest triplet state and energy in the ground state) of the compound.
Accordingly, in order to efficiently trap triplet energy of a phosphorescent dopant material in the device, first of all, a host material having larger triplet energy than that of the phosphorescent dopant material needs to be used in the emitting layer. Moreover, when providing an electron transporting layer and a hole transporting layer adjacently to the emitting layer, a compound having larger triplet energy than that of the phosphorescent dopant material needs to be used also in the electron transporting layer and the hole transporting layer. Thus, according to the typical designing idea of the organic EL device, a compound having a larger energy gap than that of a compound used in a fluorescent organic EL device is used in a phosphorescent organic EL device, thereby increasing drive voltage of the overall organic EL device.
Although a hydrocarbon compound exhibiting a high oxidation resistance and a high reduction resistance is useful for the fluorescent device, the hydrocarbon compound has a broad δ-electron cloud to render the energy gap small. For this reason, such a hydrocarbon compound is unlikely to be selected for the phosphorescent organic EL device, but an organic compound including a hetero atom (e.g., oxygen and nitrogen) is selected. Consequently, a lifetime of the phosphorescent organic EL device is shorter than that of the fluorescent organic EL device.
Moreover, device performance of the phosphorescent organic EL device is greatly affected by an exciton relaxation rate of triplet excitons much longer than that of singlet excitons in the phosphorescent dopant material. In other words, with respect to emission from the singlet excitons, since a relaxation rate leading to emission is so fast that the singlet excitons are unlikely to diffuse to the neighboring layers of the emitting layer (e.g., the hole transporting layer and the electron transporting layer), efficient emission is expected. On the other hand, with respect to emission from the triplet excitons, since spin is forbidden and a relaxation rate is slow, the triplet excitons are likely to diffuse to the neighboring layers, so that the triplet excitons are thermally energy-deactivated unless the phosphorescent dopant material is a specific phosphorescent compound. In short, in the phosphorescent organic EL device, control of the recombination region of the electrons and the holes is more important as compared with the control of that in the fluorescent organic EL device.
For the above reasons, enhancement of performance of the phosphorescent organic EL device requires material selection and device design different from those of the fluorescent organic EL device.
As a material of such a phosphorescent organic EL device, a carbazole derivative that exhibits a high triplet energy and is typically known as a hole transporting material has been used as a useful phosphorescent host material.
Patent Literature 1 (International Publication No. WO2011/132683) and Patent Literature 2 (International Publication No. WO2011/132684) each disclose that a compound provided by introducing a nitrogen-containing heterocyclic group to a biscarbazole skeleton in which two carbazoles are bonded is used as a host material in an emitting layer of a phosphorescent organic EL device. The compounds disclosed in Patent Literatures 1 and 2 are in a molecular design of well-balanced charge transportation achieved by introducing an electron-deficient nitrogen-containing heterocyclic group to a hole-transporting carbazole skeleton.
Moreover, Patent Literature 3 (International Publication No. WO2011/155507) discloses an organic EL device including an emitting layer in which a plurality of host materials are mixed in an attempt to prolong a lifetime of the organic EL device. In Patent Literature 3, combinations of various host materials to be mixed are studied.
However, in the organic EL devices using the compounds disclosed in Patent Literatures 1 and 2, prolonging a lifetime of each of the organic EL devices is a technical problem.
Moreover, a lifetime of the organic EL device disclosed in Patent Literature 3 is also desired to be prolonged.